1. Field
The subject invention is in the field of lifting surfaces particularly those incorporated in aircraft or aeronautical propulsion systems. More specifically it is in the field of wing tip devices used with aircraft wings to minimize induced drag by reducing the intensity and concentration of vorticity which trails from lifting surfaces.
2. Prior Art
The patents listed below constitute a representative listing of the prior art in this field:
______________________________________ U.S. Pat. No. Date Inventor ______________________________________ 2,576,981 12/1951 R. Vogt 4,714,215 12/1981 Jeffrey A. Judd et al 4,245,804 1/1981 Ishimitsu et al 4,205,810 6/1980 Ishimitsu 3,845,918 11/1974 White, Jr. 4,050,397 9/1977 Vanderleest ______________________________________
The drag of an aircraft wing arises from a number of sources of which that associated with the trailing vortex system is a major portion--approaching one-half the total drag for a subsonic airplane in optimum cruise flight. It has long been recognized that this so-called induced drag is directly associated with wing lift and load variation along the wing span. This condition corresponds to a flow whose primary manifestation is a vorticity sheet shed downstream of the wing trailing edge (i.e., the trailing vortex system) that is very intense near the wing tip. The induced drag depends on the following parameters in a manner conveniently expressed as follows: EQU Induced Drag=K.sub.i (lift).sup.2 /.pi.q(span).sup.2
where
q is dynamic pressure=1/2.rho.V.sup.2 PA1 .rho. is air density PA1 V is flight velocity also, PA1 K.sub.i is the induced drag factor
The induced drag factor, K.sub.i depends on the spanwise load distribution and the configuration of the lifting system. For a planar wing, the elliptic loading is optimum and K.sub.i =1.0. However, it is also known that the minimum induced drag is less (K.sub.i &lt;1.0) for configurations with increased ratios of total trailing edge length to span. Examples include monoplanes with tip winglets or end-plates, multiplanes of various types (e.g., biplane, triplane) and various forms of arched lifting surfaces, either open or closed. Also, assorted tip devices involving the use of multiple surfaces have been proposed for application to monoplanes or multiplanes. Many of the above are not particularly efficient or useful for various reasons including excessive structural weight, high loads, concomitant drag sources and operational limitations. Thus, with the exception of the monoplane with winglets, they find little use today. Several forms of winglets are currently in use for applications where span and operational space may be limited or where existing airplane configurations can otherwise benefit from their use. However it has not been generally established that winglets are preferable to or more efficient than simple wing span extensions to reduce induced drag. In many cases their relative benefit is marginal or even cosmetic. Winglets today are almost invariably attached to the wing tip in a way that results in relatively sharp corners and rapid changes in chord at their intersection, such as shown in the cited patents to Ishimitsu et al. This lack of a smooth, gradual transition results in significant departures from the optimum loading and the shedding of concentrated vorticity at the intersections. Thus the anticipated benefit of the winglet will not meet performance expectations and other adverse effects (e.g., premature stall, buffet) can result.
The object of the present invention is to provide a winglet configuration concept which includes an efficient transition between the wing and the winglet which maintains near-optimum loading over a substantial range of operating conditions thereby achieving the full drag reduction potential of the wing tip device. This blending between wing and winglet (hence, the title "blended winglet") is accomplished in a way which observes recognized structural and geometric limitations while maintaining favorable performance and operational characteristics.